How to determine which procurement law regime applies?

If the procurement for the relevant contract was started before 24 February 2025 then it will still be regulated by the Public Contracts Regulations 2015 (or equivalent)

Transitional Provisions

Generally, a procurement will be "commenced" on the date that the contract notice/advertisement is published. 

More detailed government guidance on transition is available here

Contracts whose procurement was commenced before 24 February 2025 will continue to be regulated by the Public Contracts Regulations 2015 (or, where relevant, by the equivalent Utilities/Defence/Concessions regulations).

This includes where any modifications are made to the contract on or after 24 February 2025.

Where the procurement of a framework agreement or dynamic purchasing system was commenced before 24 February 2025, then that agreement and all contracts called off under it, will remained regulated by the "old" regime - even on or after 24 February 2025.

As a result, the "old" regime will continue to apply to some contracts and call-off contracts for many years to come. 

Note that Dynamic Purchasing Systems whose procurement was commenced prior to 24 February 2025 will be deemed to terminate on 23 February 2029 (if they have not already terminated earlier).